PET/MRI

Researchers have introduced a new modality coupling into the fusion technology arena. Working in Simon R. Cherry’s, PhD, laboratory at the University of California (UC), Davis, scientists developed an MRI-compatible PET scanner designed for studying mice.

Explaining the importance of simultaneous acquisition in their paper, “Simultaneous In Vivo Positron Emission Tomography and Magnetic Resonance Imaging,” published in the March 11 issue of the Proceedings of the National Academy of Sciences, the researchers wrote: “To ensure that a subject is being imaged in the same physiologic state, and to correlate changes over time in the PET and MRI signals in response to an intervention … often requires that the data be acquired simultaneously. Simultaneous acquisition of PET and MRI data by using an integrated imaging device is, therefore, necessary to answer many important biomedical questions in dynamic, living systems.”

Cherry and his colleagues first conceptualized the PET/MRI fusion in 1996 and developed a rudimentary prototype one year later. In moving forward, they encountered several substantial challenges, not the least involving how PET and MRI systems interfere with each other and create artifacts and image distortion. The major concerns involved electromagnetic interference (EMI) and the negative impact that MRI’s magnetic field has on PET scanner detectors, which are typically based on scintillators coupled with PMTs. “These vacuum tubes are incredibly sensitive to magnetic fields and simply won’t work inside a magnet,” says Cherry.

Conversely, PET’s electrical and radiofrequency components can significantly disrupt the MRI system. To address this problem, the researchers deployed the silicon-based Avalanche photodiode detectors. “Obviously, we needed an alternative detector technology that would be much less impacted by magnetic fields. The magnetic field-insensitive photodiode detectors proved very effective. We used very short optical fiber bundles to appropriately place the photodiode detectors and PET electronics, with respect to MRI radiofrequency and gradient coils, to minimize interference. We’ve placed them in fields up to 9.4 Tesla without a problem,” says Cherry.

In this way, the researchers were able to develop a PET/MRI machine that effectively positions the PET scanner within the MRI scanner.

It’s interesting to note that with the early prototype, project participants used the PMTs. “However, we used them with very long optical fiber connections between scintillator elements and PMTs to bring the signal out of the magnet, which enabled us to position the PMTs somewhere else in the room,” Cherry says.

While this approach eliminated EMI between the PET and MRI systems, it proved impractical. PET scanner performance was poor compared with stand-alone PET scanners, and the length of the optical fibers made the system cumbersome. “So, from about 1997 to 2003, we waited for new detector technology to come along and mature to the point where it could make PET/MRI practical,” recalls Cherry. That happened when the Avalanche technology emerged in 2003. “That took us to the point where we could build the PET/MRI scanner. So the prototype we wrote about is really based on our last four years of work. We produced our first images in 2006 and have been using PET/MRI for biological research applications for the past two years,” he says.

Innovative introductions are usually followed by questions about when the development will be commercialized. It’s hard to say when PET/MRI will move to the mainstream clinical setting.

“Siemens has developed a prototype for distribution in the United States as a research tool,” reports Cherry. “But they’re not ready yet to offer it as a product.” Siemens Medical Solutions, Germany introduced the PET/MRI prototype in May 2007 in Berlin at the International Society for Magnetic Resonance in Medicine annual meeting. The system has been used for in vivo human brain imaging studies at Siemens’ U.S. medical facilities. Complete testing on the system began in late 2007.

“I’ve also heard that other vendors are busy working on PET/MRI,” adds Cherry. “When major medical companies invest money into technology, you know that products are imminent.”